Posttraumatic stress disorder (PTSD) is a common and disabling psychiatric disorder. Left untreated or under-treated, it can become a chronic condition associated with significant distress, depression, aggression, family disruption, and substance abuse. There is also accumulating evidence that combat-related PTSD is associated with an increased risk of morbidity and mortality. For the welfare of returning veterans with PTSD and their families, it is critical that this disorder is promptly identified and effectively treated. Considerable advances that have been made in the assessment and treatment of PTSD in recent years; however, psychopharmcoloical treatments have been shown to be largely ineffective for veterans with PTSD. To address this gap, this proposal seeks to test an innovative treatment approach in PTSD - pharmacological manipulation of the body's major stress system (the hypothalamic-pituitary-adrenal (HPA) axis) with mifepristone. At high doses mifepristone is a glucocorticoid receptor (GR) antagonist with peripheral and central nervous system effects, making it a compound of interest in the treatment of stress- related disorders. There is abundant evidence of enhanced GR sensitivity in veterans with PTSD which is thought to underlie some of the symptoms of PTSD and associated disturbances in mood and cognition. Thus, blockade of the GR receptor with mifepristone may target unique aspects of PTSD and lead to clinically meaningful improvement in symptoms and cognition. There is preliminary evidence that short-term mifepristone treatment has sustained beneficial effects on mood, cognition and sleep disturbance in some neuropsychiatric conditions (major depression, bipolar disorder, primary insomnia). That there can be sustained clinical and neuropsychological effects of mifepristone and normalization of basal HPA axis activity after drug discontinuation in these disorders, has led to the view that mifepristone's actions include re- calibration of a dysregulated HPA axis. Accordingly, we propose to study the effects of mifepristone in veterans with chronic PTSD to determine if it is efficacious in improving PTSD symptoms and associated clinical outcomes. To better understand the mechanism of action of mifepristone we propose to assess the effects of mifepristone on HPA axis activity and their relationship to treatment outcome and clinical response. To achieve these objectives, we propose to conduct a Phase IIa, multi-site, double-blind, placebo- controlled trial of mifepristone in veteran outpatients with military-related PTSD through the VA's Cooperative Clinical Trial Award program. We propose to enroll 135 unmedicated male veterans with military related PTSD at four VA sites. Eligible veterans will be randomly assigned in parallel groups to treatment with 600 mg/day mifepristone, 1200 mg/day mifepristone, or placebo for one week and followed for up to three months. Using statistical selection theory, we propose to determine whether 600 mg or 1200 mg of mifepristone yields a sufficiently high proportion of clinical responders after one month to warrant more extensive and definitive research as part of a Phase III trial. Secondarily, we seek to determine the effect of two different doses of mifepristone compared to placebo on the trajectory of CAPS-2 scores and the time to addition of rescue medication, as well as compare rates of adverse events and serious adverse events across the three groups. We will also describe the effects of mifepristone on several other clinical parameters including PTSD symptomology, depression severity, sleep quality, and functional impairment. Several measures of neuroendocrine functioning will also be obtained to explore the relationship of plasma cortisol and ACTH levels to clinical response and the time to addition of rescue medications.